Ethanol could be an attractive fuel for use in SOFC combined heat and power plants, for instance those plants intended for use as auxiliary power units for trucks and marine applications. Potentially the fuel processing steps in such a plant could be very simple ultimately being only evaporation of the ethanol and injection into the anode chamber of the SOFC.
This approach would, however, lead to a number of problems and disadvantages:
Saunders, G. J. et al. (Formulating liquid hydrocarbon fuels for SOFCs, Pages 23-26, from Journal of Power Sources Volume 131, Issues 1-2, Pages 1-367 (14 May 2004)) show that dry ethanol is very prone to form carbon at conditions prevailing in the anode chamber of the SOFC with the most active Ni-cremates as anode material, resulting in deactivation of the SOFC after a few hours of operation. It is well known that it is very difficult to avoid carbon formation from ethanol on Ni containing catalyst under steam reforming conditions where ethanol is reacted with steam. The reason for this being dehydration of ethanol to ethylene which then polymerises. The problem of coking involved with ethanol reforming catalysts is described, for example, by Haga et al. in Nippon Kagaku Kaishi, 33-6 (1997) and Freni et al. in React. Kinet. Catal. Lett. 71, p. 143-52 (2000). Thus, reforming of the ethanol in the anode chamber by adding water (internal reforming) is not a simple way to alleviate the problem with carbon formation.
Carbon formation on steam reforming catalysts and in a SOFC plant can also take place by the following reversible reactions:CH4C+2H2(−ΔH298=−74.9 kJ/mol)  [1]2COC+CO2(−ΔH298=172.4 kJ/mol)  [2]
Reaction [2] is known as the Boudouard reaction. Ethanol can decompose to CO according to reaction [3]CH3CH2OHCO+H2+CH4(−ΔH298=−51.3 kJ/mol)  [3]
As CO is quite reactive, it is important to know the temperature and gas composition ranges, where reaction [2] does not occur. This can be studied using “the principle of the equilibrated gas” assuming both methanation/steam reforming (reaction [4]) and the shift reaction (reaction [5]) to be in equilibrium, as further described by Nielsen, J. R. (Catalytic Steam Reforming, Springer Verlag, Berlin 1984).CH4+2H2OCO2+4H2(−ΔH298=−165.0 kJ/mol)  [4]CO+H2OCO2+H2(−ΔH298=41.2 kJ/mol)  [5]
Sasaki, K. and Teraoka, Y. (Equilibria in Fuel Cell Gases Pages 1225-1239 from Solid Oxide Fuel Cells VIII (SOFC VIII) Proceedings Volume 2003-07) have studied the amount of water needed to avoid carbon formation.
A further disadvantage of using ethanol as direct fuel in an SOFC compared to using methane is related to the heat of reactions when steam reforming this fuel. Steam reforming of methane is given in equation 4 and the reforming reactions for ethanol is given in equation 6:CH4+2H2OCO2+4H2(−ΔH1023=−191.4 kJ/mol)  [4]CH3CH2OH+3H2O2CO2+6H2(−ΔH1023=−209 kJ/mol)  [6]
Reforming of the fuel in the anode chamber (internal reforming) helps to cool the stack due to the endothermal nature of the reforming process. However, the heat of reactions for ethanol reforming are much less endothermic (pr H2 produced) than methane steam reforming, therefore the cooling of the stack provided by steam reforming of ethanol is less effective.
The fuel processing method of the invention describes a process lay-out where all the above problems are overcome by adiabatically converting ethanol into a mixture of methane, H2, CO, CO2 and water.
It is an objective of the invention to provide a fuel processing method for solid oxide fuel cells, whereby the fuel ethanol is adiabatically converted to a mixture of methane, H2, CO, CO2 and water before conversion in a solid oxide fuel cell.